(1) Field of the Invention
The present invention relates to a color image display control apparatus for receiving a composite color picture signal and processing color image data included in the signal.
A color image display control apparatus is used for processing an original image included in a composite color picture signal, for example, to improve quality of the image, to superimpose characters, or to insert a small picture area into the original image area, etc.
Owing to recent progress in digital signal processing technology, and to enable image processing by computer control, the image processing is usually carried out in the form of digital data.
(2) Description of the Related Art
FIG. 1 shows an outline of a conventional construction of a color image display control apparatus.
In FIG. 1, reference numeral 1 denotes an analog to digital converter, 2 denotes a Y/C separation circuit, 3 denotes a digital image processing circuit, 4 denotes a matrix circuit, 5 denotes a digital to analog converter, and 6 denotes a display apparatus.
The analog to digital converter 1 converts a color picture signal in an analog form included in a composite color picture signal to a digital form at each cycle of a sampling clock.
The Y/C separation circuit 2 separates a luminance signal Y and color difference signals R-Y and B-Y, or I and Q, from the composite color picture signal, where R denotes a color signal "red", B denotes a color signal "blue" and Y denotes a luminance signal, I denotes an I signal which is defined by I=-0.27 (B-Y)+0.74 (R-Y), and Q denotes a Q signal which is defined by Q=0.41 (B-Y)+0.48 (R-Y).
The digital image processing circuit 3 processes digital image data.
The matrix circuit 4 composes color signals from the luminance signal and the color difference signals.
The digital to analog converter 5 converts processed digital image data to an analog form.
The display apparatus 6 displays an image corresponding to the processed image data.
All of the above components are well-known in the field of digital image processing technology.
In the above construction, a conversion of an analog color picture signal included in the composite color picture signal to digital form, processing of the digital image data, and a conversion of the processed digital data to an analog form, are carried out.
A standard composite color picture signal, which conforms to a standard system for television transmission, for example, NTSC system, PAL system, etc., includes vertical and horizontal synchronizing signals, a luminance signal, color signals, and a color burst signal. The color burst signal gives a reference phase for demodulating color signals.
When converting an analog color picture signal to a digital form in color image display control apparatuses, usually, a clock signal having a frequency n (n is an integer) times of the color sub-carrier f.sub.sc and synchronized with the color burst signal, is used as a sampling clock.
On the other hand, a position where each sampled data in a horizontal line is displayed, is determined based on the timing of the horizontal synchronizing signal because the scanning of each horizontal line is triggered at the timing of the horizontal synchronizing signal.
Generally, in standard composite color picture signals, for example, the television signals sent from the public broadcasting station, a phase difference between the color burst signal and the horizontal synchronizing signal is a predetermined constant.
However, when a composite color picture signal is played back by a popular video tape recorder or a laser disk recorder which was manufactured for personal use, the phase difference between the color burst signal and the horizontal synchronizing signal is not maintained to be a constant anymore, due to a mechanical inaccuracy of the structure of the video tape recorder or a laser disk recorder for personal use. The inaccuracy is commercially inevitable at present because achieving high mechanical accuracy causes an increase in cost.
Thus, the variation in the phase difference between the color burst signals and the horizontal synchronizing signals, causes a dislocation of image on a horizontal line, and therefore, causes a distortion in the horizontal direction of a displayed image.
FIGS. 2(A) and 2(B) show a variation of phases of sampled image data according to a variation of phase differences between a horizontal synchronizing signal and a sampling clock, which variation results in dislocations of image data in the horizontal direction on a display screen.
In FIGS. 2(A) and 2(B), an original image data on a horizontal line before being sampled is shown on the top.
If a phase difference from a horizontal synchronizing signal is equal to zero in the line A, as shown in FIG. 2(A), when the above original image data shown in the top is sampled by the sampling clock, the sampled image data "A" is obtained.
If a phase difference from a horizontal synchronizing signal is equal to .theta. in the line "B", as shown in FIG. 2(B), when the same original image data shown in the top is sampled by the sampling clock, the sampled image data "B" is obtained.
As shown FIGS. 2(A) and 2(B), when a phase difference between the horizontal synchronizing signal and a sampling clock varies, the phase of the sampled image data on the horizontal line varies accordingly.
FIG. 3 illustrates the distortion of the image caused by the above difference between the horizontal synchronizing signal and the sampling clock shown in FIGS. 2(A) and 2(B). As explained above, the distortion originates from the difference between the horizontal synchronizing signal and the color burst signal.
Typically, the cycle time 1/f.sub.sc of the color burst signal is about 280 nsec, the cycle time (1/4f.sub.sc when a sampling clock having a frequency 4f.sub.sc is used) of the sampling clock is about 70 nsec, and the typical range of the variation of the phase difference between the horizontal synchronizing signal and the color burst signal is about 10 to 20 nsec. The distortion in the horizontal direction (shown by "d" in FIG. 3) due to the variation of the phase difference about 10 to 20 nsec, amounts to about a few millimeters. This amount of distortion is well visible and greatly deteriorates the displayed image.
In the prior art, some attempts have been carried out to solve the above problem.
In the attempts in the prior art (T. Kurita et al., "IDTV apparatus by SCAD system" in the 1989 National Convention Record of the Institute of Television Engineers of Japan, pp 309-310, No. 13-3, R. Kobayashi, "IDTV apparatus by SCAD (sub-carrier A/D) system" in 1989 National Convention Record of the Institute of Television Engineers of Japan, pp 311-312, No. 13-4), first, it is determined whether a received signal is a standard composite color picture signal wherein a phase difference between the color burst signal and the horizontal synchronizing signal is a predetermined constant, or non-standard composite color picture signal wherein a phase difference between the color burst signal and the horizontal synchronizing signal varies. Next, if the signal is a standard composite color picture signal, the color image display control apparatus is operated with a system clock synchronized with the color burst signal. Or if the signal is a non-standard composite color picture signal, the color image display control apparatus is operated with a system clock synchronized with the horizontal synchronizing signal.
However, in the above attempts, a circuit for detecting the standard/non-standard composite color picture signals, additional constructions including, for example, a circuit for generating a sampling clock synchronized with the horizontal synchronizing signal, a digital circuit for demodulating color signals operating by the sampling clock synchronized with the horizontal synchronizing signal, or an analog circuit for demodulating color signals which operates regardless of sampling clock, etc. must be provided. Therefore, the above attempt greatly increased the scale of the circuits.
Further, when a circuit operating with a sampling (system) clock and another circuit operating with another sampling (system) clock are incorporated in a system, an interference between the two different system clocks, which causes harmful noises, occurs.